(i) Technical Field
A certain aspect of the embodiments discussed herein is related to a method for fabricating a semiconductor device.
(ii) Related Art
Conventionally, there is known a semiconductor device formed by stacking, on a semi-insulation substrate, GaN-based semiconductor layers such as a buffer layer, a GaN layer and an AlGaN layer. As an exemplary GaN-based semiconductor device, there is known an electronic device having an FET (Field Effect Transistor) structure that utilizes carriers with very high concentration generated at an interface between GaN-based semiconductor layers.
A silicon carbide (SiC) may be used as a substrate for the electronic devices using group III nitride semiconductors as described above. However, SiC is expensive and has difficulties in realizing a large scale wafer and reducing defects such as micropipes. Therefore, it is required to use another substrate. The use of silicon for the substrate is considered. Silicon is less expensive and easily realizes a large-scale wafer and a few crystal defects. It is to be noted that Si of the substrate reacts with Ga. This reaction may make it difficult to grow the GaN-based semiconductor layer having a desired crystal structure. Thus, a buffer layer may be interposed between the Si substrate and the GaN-based semiconductor layer. The buffer layer may be an AlN layer, for example. The buffer layer prevents impurities such as Ga from diffusing into the Si substrate. Japanese Patent Application Publication No. 2008-166349 describes a process of growing a GaN-based semiconductor layer on the substrate and adjusting the Al composition ratio of the AlGaN layer.
However, the crystal quality of the AlN layer may affect the semiconductor device.